The study investigates the effects of the Argumentation-Based Inquiry (ABI) approach in dual-language (DL) courses on oral presentation (OP) skills and willingness-to-communicate (WTC) among EFL high school students. Using a mixed-methods research approach with an embedded design model, the study emphasizes the quantitative approach (QUAN) for an instructional experiment and the qualitative (QUAL) approach to explore student and teacher perceptions. A quasi-experimental design randomly assigned students from two schools into three groups: Baseline Group, Experimental Group 1 (ABI), and Experimental Group 2 (ABI with a self-regulated learning approach), involving EFL Grade 11-12 students (n = 84). Data were analyzed using MANCOVA and theme analysis, with interviews of students and teachers. Results indicated that Experimental Group 2 consistently achieved the highest OP scores (content knowledge, organization, videotaped delivery, language control, and fielding questions) and WTC preparedness across three engagement and motivation levels, as compared to Experimental Group 1 and Baseline Group. Interviews supported the feasibility of implementing explicit OP skill instruction in DL courses. Overall, EFL high school students at different proficiency levels demonstrated improvements in both formal oral presentation (OP) skills and willingness to communicate (WTC) in English. The findings suggest that the ABI approach in dual-language (DL) courses can effectively enhance the OP skills and WTC of EFL learners.